1. Field of the Invention
The present invention relates to a substrate for mounting an optical semiconductor element, a manufacturing method thereof, an optical semiconductor device using such a substrate, and a method of manufacturing an optical semiconductor device.
2. Description of the Related Art
In recent years, in addition to reductions in size and weight, there have been advances in the performance and multifunctionality in the field of electronic equipment, and electronic components have come to be mounted at high densities in electronic equipment. What are called SMDs (Surface Mount Devices), which can be mounted on the surface of print boards, have come to be the mainstream of electronic components used in electronic equipment; SMDs can be connected to wiring patterns on print boards by reflow soldering and other methods, and are used in various applications.
For example, SMD type LEDs (Light-Emitting Diodes) are electronic components used in the field of optical semiconductors. LEDs are optical semiconductor devices which combine optical semiconductor elements and fluorescent materials, and are attracting attention as light-emitting devices which are energy-efficient and have long lifetimes. In recent years there has been vigorous research and development of optical semiconductor elements relating to increasing the brightness, boosting the output, and shortening the emission wavelength, among other performance enhancements; and great hopes are being placed on LEDs as next-generation light sources.
General SMD-type LEDs are explained using FIG. 1. FIG. 1 is a perspective view showing an example of the structure of a conventional SMD-type LED. As shown in FIG. 1, the SMD-type LED 100 substantially comprises an LED package substrate 110, and an optical semiconductor element 120 mounted in a depressed portion thereof. The LED package substrate 110 comprises a metal substrate 112, and a resin substrate 116 provided thereupon with an adhesive sheet 114 intervening. On the upper face of the metal substrate 112 is formed a pair of connection terminals (not shown) for electrical connection to each of the electrodes of the optical semiconductor element 120; the surfaces of the electrodes are treated with silver plating or similar.
The resin substrate 116 is configured so as to function as a reflector, and in the center portion thereof is formed a penetrating hole 118, comprising a circular upper opening 116a and lower opening 116b as well as an inner wall 116c. The penetrating hole 118 forms a cup-shape depressed portion as a result of connection of the resin substrate 116 with the metal substrate 112 with the adhesive sheet 114 intervening. The portion of the adhesive sheet 114 corresponding to the lower opening 116b is removed.
Next, an example of a method of manufacture of a SMD-type LED is explained. FIG. 2A and FIG. 2B are summary diagrams explaining processes in a conventional method of manufacturing an LED package in which a plurality of optical semiconductor elements are arranged in a matrix. By means of such a method, a plurality of LEDs can be manufactured at one time.
In the specific processes, as shown in FIG. 2A, an LED package substrate 210 is manufactured by applying pressure and heat to a resin substrate 216 serving as a reflector and bonding this onto a metal substrate 212 on which are mounted in advance at prescribed positions optical semiconductor elements 220, with an adhesive sheet 214 in which opening portions 214a have been provided in advance, intervening. The positions of the opening portions 214a of the adhesive sheet 214 correspond to the positions of the lower openings 216b of the resin substrate 216. The LED package substrate 210 obtained in this way is then cut into individual LEDs along dicing lines 230 in two directions, as shown in FIG. 2B, to finally obtain SMD-type LEDs 100 as shown in FIG. 1.
In the above-described SMD-type LED, as the performance of the optical semiconductor element is enhanced, there is a tendency for the heat and optical energy generated by the element itself to increase. The heat generated by the optical semiconductor element causes the temperature of the optical semiconductor element and the optical semiconductor device to rise. In particular, as shown in FIG. 3, in the case of an SMD-type LED 300 which employs an insert-type lead frame as the base member, lead terminals are mounted on a device side face as electrical external connection terminals. In the figure, the reference number 312 denotes a metal substrate, 312a is the lead frame, 316 is a reflector, 320 is the optical semiconductor element, 330 is a wire bonding wire, 342 is a transparent sealing resin, 344 is a fluorescent material, and 350 is a die bond material. In such an SMD-type LED, particularly when no measures are taken to deal with heat, the path for dissipation of heat generated by the optical semiconductor element 320, which is surrounded by the transparent sealing resin 342 having poor thermal conductivity, extends to the lead frame 312a and the external electrodes (not shown) connected to the lead frame. Hence because the distance from the light-emitting element which is the heat source to the external electrodes is long, thermal resistance is high, and due to this structure, increases in the temperature of the optical semiconductor element are unavoidable.
Increases in the temperature of the optical semiconductor element cause degradation of the optical semiconductor element, and induce reduced brightness, shortened lifetimes, and other declines in the performance and reliability of the optical semiconductor device, and so are undesirable. As methods to suppress increases in temperature, materials with high thermal conductivity may be used in the substrate on which the device is mounted or in constituents of the optical semiconductor device, or members may be designed to have structures which reduce thermal resistance. However, when a heat sink or metal/ceramic package with high thermal conductivity or similar are used as members employing materials with high thermal conductivity, the overall number of components as well as the number of manufacturing processes are increased, which tend to increase costs and detract from manufacturing productivity.
Given such circumstances, methods have been disclosed in which a thermoplastic resin with excellent heat dissipation, heat resistance, ultraviolet light resistance, and other characteristics is used to form, by various methods, the reflector portion, that is “depressed portion”, in the substrate for mounting an optical semiconductor element.
In Laid-open Japanese Patent No. 2006-140207, a method is disclosed for manufacture of a substrate for mounting optical semiconductor element, in which a specific thermosetting resin compound for optical reflection, having heat resistance and light resistance, is used to form a reflector portion by a transfer molding method. An optical semiconductor element mounting substrate manufactured according to the method disclosed has good adhesion to a wiring board formed from a resin composition, and has excellent reliability due to the minimal occurrence of warping caused by the resin composition.
Further, in Laid-open Japanese Patent No. 2007-142253, a method is disclosed in which, in manufacturing a wiring board for a light-emitting element, a reflector portion is formed directly by molding, without the intervention of an adhesive layer, on a fiber-containing resin board on which is formed a wiring pattern, to manufacture a molded board.
According to the above-described method, a depressed portion, which becomes the area for mounting optical semiconductor element, can be molded integrally with a printed circuit board. However, when this board is formed as a large molded object of size 20 mm2 or larger, in which are arranged a plurality of optical semiconductor elements in a matrix as shown in FIG. 2A and FIG. 2B, differences in linear expansion rates give rise to stress-induced warping, and it becomes difficult to execute subsequent processes, among other problems. Also, when forming a depressed portion by transfer molding of a resin composition, resin burrs may occur at the lead terminal surface or other external connection terminal portions, and there is a strong possibility that a process to remove such burrs must be added. Further, there is a tendency for resin to be not sufficiently filled between circuit wires in circuits in which optical semiconductor elements and external connection elements are arranged in a complex manner for electric input. If resin is not sufficiently filled between circuit wires and thereby unfilled portions of resin exist, when subsequently filling the depressed portion with transparent sealing resin, transparent resin may leak from the unfilled portions, so that filling faults may occur, and execution of subsequent manufacturing processes becomes difficult.
Hence it has been deemed necessary to perform further development related to materials comprised by members of optical semiconductor mounting boards, optical semiconductor devices and similar, as well as to design of member structures. And hereafter, when manufacturing high-output, high-brightness LEDs, it is anticipated that numerous optical semiconductor elements will be mounted on a substrate. And as the number of mounted optical semiconductor elements increases, the number of external connection terminals for connection of elements to external electrodes must also be increased. Hence a structure for mounting optical semiconductor substrates enabling easy connection of elements to external connection terminals, as well as an efficient and low-cost method of manufacturing such substrates, is desired.